1. Field of the Invention
The present invention relates to an output voltage control apparatus for a vehicular AC generator mounted on a vehicle such as an automobile.
2. Description of the Related Art
Since a vehicular AC generator is driven by an engine mounted on a vehicle, when a control is not performed, the output voltage of the generator varies according to the engine speed. In order to control the generator output voltage within a specified voltage range even in the case where the engine speed is increased, an output voltage control apparatus is used. As the output voltage control apparatus for the vehicular AC generator, a fixed frequency type output control apparatus is disclosed in, for example, U.S. Pat. No. 4,388,586. This fixed frequency type output voltage control apparatus repeats a control period of a specified time width, controls an ON period of a field current of an AC generator in each of control periods, and adjusts a generator output voltage.
The fixed frequency type voltage control apparatus includes, for example, a flip-flop, a voltage detection circuit, and a comparison circuit. The flip-flop starts the ON period of the field current at a start time point of each of the control periods of the specified time width. The voltage detection circuit outputs a detection voltage corresponding to the generator output voltage. The comparison circuit compares the detection voltage with a reference voltage, and performs the control so that when the detection voltage is lower than the reference voltage in each of the control periods, the flip-flop continues the ON period, and at the time point when the detection voltage becomes higher than the reference voltage, the flip-flop is inverted and the ON period is ended. Since the fixed frequency type output voltage control apparatus controls the field current in synchronization with each of the control periods of the specified time width, there is a merit that the control can be easily made by, for example, a microcomputer.
Since the vehicular AC generator is provided with a rectifier unit to convert the AC output voltage into DC voltage, the generator output voltage obtained at the output side of the rectifier unit includes a ripple component due to the rectification operation of the rectifier unit. Since the voltage detection circuit is also connected to the output side of the rectifier unit, the detection voltage outputted by the voltage detection circuit also includes a ripple component. In the detection voltage including the ripple component, since the ripple component brings an effect of increasing the instantaneous value of the detection voltage, and consequently, the ON period is controlled to be short, the generator output voltage is controlled to be a relatively low value with respect to the reference voltage, and there arises a disadvantage that the voltage value of the generator output voltage can not be made sufficiently large.